Physiology Question on Q & R Waves

[AnthonyM83]

I'm doing a little self-study on EKGs, but stuck on a little point.

The way I'm understanding it is Q-waves in lateral leads (I, III, aVL, V5, V6) are there b/c of depolarization from the septum toward the right side (thus away from lateral leads, thus negative wave). But the text also says left side depolarization happens first.

Soo, when reading the strip how does the Q wave end up coming BEFORE the R wave in the lateral leads???

My only conclusion is that I'm misunderstanding how Q and R waves are formed or where the leads actually are in relation to parts of the heart.

(BTW, to whoever responds, mentally, I'm thinking of everything as a movie and seeing electrical currents passing through the heart toward and away each lead...rather than thinking in terms of "R wave equals ventricular depolarization"...I'm trying to visualize exactly where in the heart current is going in relation to the various leads.)

[AnthonyM83]

In addition:

My text is kind of vague on S-waves. I want to be sure S-waves are in effect the "opposite" of R waves, in that the leads are detecting the ventricular depolarization going toward the opposite side of the heart.

[AZCEP]

A few definitions first.

Q-wave: the first negative deflection from the isoelectric baseline following the P-wave

R-wave: the first positive deflection following the Q-wave

S-wave: the negative deflection following the R-wave, before the T-wave

Now, the physiology of the situation.

The AV node relays the stimulus into the Bundle of His and right and left bundle branches. The left will receive the impulse first, due to the right bundle branching further away from the AV node. This causes the septum to depolarize from the left to the right, giving a negative Q-wave deflection in the lateral leads (I, aVL, V5 & 6).

Once the impulse has moved through the septum, it will then reverse direction and move left. The movement should be from left-->right-->left, with the repolarization following a similar direction. The left ventricle will depolarize slightly ahead of the right because the impulse is transmitted to the left side first. It is this ever so slight asynchrony that allows the right ventricle to fill and causes a more effective contraction.

[AnthonyM83]

So, you're saying what the Q wave is representing isn't actually the impulse itself traveling left to right, but rather the difference (in voltage) from the left to the right side. Since left gets impulse first, then right get impulse, it reads AS IF an impulse were travelling in left to right direction (when in reality, both septum impulses are travelling downward and slightly to left). Would this be correct? Also, depending on whether lead is looking at the heart from left or right, it's either a Q wave or small R wave.

(PS I'm also referencing this picture: http://health.yahoo.com/media/healthwise/nr551740.jpg)

Then, as the lower septum and apex get depolarized, it creates either a tall R wave (if leads are looking from the left) or S wave (if looking from right). Then, as it travels up the sides of the ventricle, upwards, shouldn't you get a negative wave in V4-6? (these are the ones to the left of septum, right...) but instead they show a continuation of the R wave...only VF, II, and III show an S wave.

[fiznat]

So, you're saying what the Q wave is representing isn't actually the impulse itself traveling left to right, but rather the difference (in voltage) from the left to the right side.

Yes. This is true of all EKG tracings. I'm not sure if you've taken physics or not, but basically the readout you get on the EKG paper is a representation of a vector quantity. What that means that all lines (be they up/down, narrow/wide) are illustrations of the NET electrical direction. If you have a 2 volts going upward and 10 volts going downward, your net movement is 8 volts to the negative- and will result in a negative tracing. It has to be this way because the movement of these deplorizations would be too complicated (and probably unimportant) to measure in perfect detail. Instead, we just calculate the average (net) direction and magnitude of the movement.

Just to confirm what we're talking about here though: are you asking about physiological or pathological q waves? The two arise from different circumstances, AZCEP has explained only the former.

[AZCEP]

So, you're saying what the Q wave is representing isn't actually the impulse itself traveling left to right, but rather the difference (in voltage) from the left to the right side.

All of the wave forms on a surface ECG represent some degree of electrical activity. In the case of the Q-wave, the impulse is spreading from the left side of the septum (Left Bundle Branch) to the right. This movement is viewed as the positive deflection in V1, and a negative in V6.

Since left gets impulse first, then right get impulse, it reads AS IF an impulse were travelling in left to right direction (when in reality, both septum impulses are travelling downward and slightly to left). Would this be correct?

During the period of time that the septum is depolarizing, the stimulus is moving to the right. Once the septum is entirely depolarized the stimulus then moves back to the left. Check the third image in my previous post. Also keep in mind that this movement takes all of 0.01-0.04 seconds normally, so capturing the event is near impossible. The movement is almost instantaneous.

Also, depending on whether lead is looking at the heart from left or right, it's either a Q wave or small R wave.

Careful that you don't confuse yourself with what "bigger" and "smaller" mean when looking at an ECG. The view, or axis, of a given lead will change the overall size of the deflections. The more negatively deflected, the smaller, the wave is. The more positive, the larger. If a given lead has a parallel axis to the average movement of the impulses, the vectors that fiznat brought up, the deflections will be smaller than the deflections in a lead that is perpendicular. The larger the deflections, the more information is being delivered to the lead. The more information to the lead, the more accurate the nature of the event.

Then, as the lower septum and apex get depolarized, it creates either a tall R wave (if leads are looking from the left) or S wave (if looking from right). Then, as it travels up the sides of the ventricle, upwards, shouldn't you get a negative wave in V4-6? (these are the ones to the left of septum, right...) but instead they show a continuation of the R wave...only VF, II, and III show an S wave.

You are speaking of the R and S-wave progression. In a normal heart, the QRS is almost entirely negative when viewed in lead V1. Conversely, the QRS is almost entirely positive when viewed in V6. Somewhere between V3 and V4, the QRS will be precisely biphasic. As you look at where the leads are placed this will make sense. With the movement from left-->right-->left, the information will move away from V6 and toward V1 initially. Then the infomation moves almost entirely away from V1 toward V6.

[AnthonyM83]

Gotcha. That's making sense now, guys.

Last thing I want to be sure about:

Are the R waves in V1 and V6 from different sources?

The text said V1 registers a small positive deflection as the charge moves from left to right across the septum. As the charge moves through the ventricles, that would create a V1 S-wave...so basically that "small positive deflection" is all the R wave you'll see in V1. And that same left to right across the septum is what produces a Q wave in the left leads like V4. Whereas the R wave in the left leads are from the ventricles depolarizing (not the septum). Would this be an accurate explanation?

And yes, right now I'm just talking about normal physiological waves, not pathological...that's NEXT chapter 8)

[AZCEP]

NO.

The waves that are formed are the same in all the leads. The view that you are looking at them changes. Some will be more pronounced than others, but the ECG is not reading information coming from different sources.

[AnthonyM83]

Okay...then I'm trying to make sense of this section:

"	 The left side of the septum depolarises first, and the impulse then spreads toward the right. Lead V1 lies immediate to the right of the septum and thus registers an initial small positive deflection (R wave) as the depolarisation travels ward the lead.



	 When the wave of septal depolarisation travels away from the recording electrode, the first deflection inscribed is negative. Thus small "septal" Q waves are often present in the lateral leads, usually leads I, aVL, V5, and V6."

I read that and then looked at a sample V1 QRS given. It had a small R wave. Then I looked at a sample V5 QRS given. It had a tall R wave.

I understand that R waves get larger as leads go farther left, but if V1 is to the right of the septum, what physiological event is it getting its R wave from? It must be that small septal deflection toward V1 (described in first quoted paragraph), because that's the only impulse going toward V1 (which is needed to create a positive wave).

Now, when they explain the Q waves in the left leads (second paragraph of quoted text), it seems like they're talking about about that same septal depolarization from left to right, but since the leads are on the opposite side, they register negatively on the EKG as Q waves.

The only way I can make sense of those two paragraphs is that depending on electrode placement and if impulse is going toward/away from it, the septal depolarization reads as either pos. R or neg. Q.

Where else would V1 get a positve R wave...it can't be the ventricles depolarizing, because that would create a negative wave (such as the S wave).

Now, I'll totally believe that I'm wrong, but I don't understand how else to make sense of what the text is saying.

I appreciate the help guys...I really want to have a conceptual understanding of this and not just memorize each thing.

[AZCEP]

It sounds like you are trying to make more of the situation than is actually there.

WARNING: Do not look at the following image as anything other than a single event in several different leads. There is a lot going on, and you will only confuse yourself further if you do.

The hexaxial system in the top left shows the axis of this particular problem. The diagram of the heart shows the direction the vectors are following in sequence. The ECG pieces show how the complexes will look when you view them on the monitor/strip.

Now, as I said, this picture is of one event. The lead, or camera, is looking at different parts of the heart and showing you the image of what is happening from the given lead's perspective.

Consider the positive electrode of a lead to be the camera, okay? Now, what will happen to the image if it gets closer, or comes toward the camera? It will get bigger, or in an ECG's case more positively deflected. Knowing this, what happens when the image goes away from the camera? It will become smaller, or more negative. If the camera is in one position, and the image passes across it's field of view, what do you think will happen? It doesn't really change in size.

Another way to look at this would be to watch cars go down the street. If you stand in front of one, it gets bigger. Stand behind one, it gets smaller. Stand off to the side, and don't move your visual field, and the car stays the same size while it is in your visual plane.

The ECG gathers the information that presents to it. If the information moves toward a lead, it becomes...? If the information moves away from a lead, what happens? An interpreter of the information would expect certain things to happen in a healthy heart, and when those things aren't there can identify which part of the cardiac anatomy is being effected. This is what makes the 12 lead so useful, and intimidating to some.